The present invention relates generally to the field of marine seismic cables, and, more particularly, to a structure and method of communicating with a level control device coupled to the cable, known in the art as a xe2x80x9cbirdxe2x80x9d.
Solid and semi-solid seismic exploration cables for the marine seismic exploration industry have been in development for some time. Marine seismic cables are central to the acquisition of data for seismic exploration at sea and the development of solid or semi-solid cables has been directed to eliminating the well-known and documented problems associated with fluid filled cables. For example, the fluid in a fluid-filled cable effects the buoyancy of the cable, which is also affected by the temperature and salinity of the water in which the cable towed. Further, oil-filled cables may leak. The outer jacket of a seismic cable of the prior art is prone to rupture or tearing. This is particularly undesirable because this exposes the internal electrical components to seawater, and disrupts the buoyancy of the cable. Thus, a totally solid seismic cable would eliminate such drawbacks of fluid-filled cables.
Depth control devices on the cable, commonly known as birds, are attached to the cable at intervals in order to maintain the cable at a predetermined depth. Such birds are typically attached to the cable through a collar arrangement firmly mounted to the cable. In such systems, communication is coupled through magnetic induction coupling between a communication coil within the fluid in the cable and secondary coils in the bird. However, streamer communications coils are typically constructed with the wiring wound around single ferrite core. Such bulky and fragile devices are not suitable for installation into a solid streamer cable.
Other external devices such as acoustic location modules, compasses, and depth transducers may also use the communication coil for telemetry. This wireless communication system, commonly known in the art, eliminates the need for a physical connection between the communication coil inside the cable and the external device, thus better ensuring water-tight enclosure of the cable. The prior art wireless communication system used with a kerosene-filled cable, or non-solid cable, includes a single coil that is attached with plastic xe2x80x9ctiesxe2x80x9d to the bundles of wires which make up a center core of such a cable.
A possible solution to the problem of communication between a device external to a marine seismic cable and the cable was suggested in U.S. Pat. No. 5,923,616 to Badger et al. A system was suggested including a solid marine seismic cable assembly including communication coil housings which clamp around a cable in a spaced-apart relationship. Each housing contains at least two communication coils adapted for communicating with an adjacent external device. Two of the communication coils, whether or not adjacent to one another, are offset at least 90xc2x0 from each other about the axis of the cable. Each external device has a unit which communicates with the communication coil via magnetic induction signals.
A major deficiency of this type of construction is that it requires the coils in each housing to be wired together. This increases the complexity of manufacture and compromises reliability in the system as a whole.
The coils of the communication system are offset at least 90xc2x0 from each other so that, as the external device revolves around the housing, magnetic coupling is maintained. However, the single coil arrangement often results in low signal strength with poor coupling. Further, having only a single coil in communication at one time necessarily means that one coil is superfluous. Thus, there remains a need for a system which provides greater signal strength with all communication coils effectively employed. Such a system should provide for radial alignment, despite the movement of the cable and the external devices.
The present invention addresses these and other needs in the art by providing a communication housing, especially for level control devices, comprising an upper housing half and a lower housing half, coupled together around the cable. The upper housing half includes a plurality of wells, for example three wells, each adapted to receive a communication coil. The lower housing half has no such wells, and there is no interconnect wiring between the halves.
The communication coils are wound in series to reinforce the strength of the communication signal. Each of the individual communication coils is preferably formed in two segments, joined together at a flexible joint to reduce the likelihood of breakage of the coil core as the cable is subjected to bending under load as it is wound onto a takeup reel aboard the exploration vessel.